Southeastern Apple Production

PLANT PATHOLOGY

Armillaria

Armillaria mellea and Armillaria tabescens (Clitocybe tabescens)

Armillaria root rots, also known as mushroom root rots, are caused by two species of Armillaria, Armillaria mellea and A. tabescens (Clitocybe tabescens). Armillaria mellea is also known as the oak root rot or the shoestring root rot fungus.

Symptoms

Aboveground symptoms are similar to the other root rots, but early symptoms are often unilateral, affecting one or two scaffold limbs that may die back (Fig. 1). Symptoms on roots of trees affected by the two Armillaria species are similar. The fungi grow within and beneath the bark and appear as white to slightly yellow mycelial growth (Fig. 2). This characteristic differentiates the mushroom root rots from white root rot that is characterized by white mycelium visible on the outside of affected roots (Fig. 3). Fan-shaped mycelial mats are often produced between the bark and wood. Perforations may be observed in the mycelial mats produced by A. tabescens (Fig. 4). Armillaria mellea is distinguished from A. tabescens by the presence of dark brown to black, root like structures known as rhizomorphs (Fig. 5), which are often found on dead roots, on the surface of live roots, or growing out into the soil. In the fall following rain, yellow to yellow-brown to brown mushrooms are often produced in clusters at the base of affected trees (Fig. 6). The stems (stipes) of the mushrooms of A. mellea (Fig. 7) have a ring of tissue around them known as an annulus, which is absent in A. tabescens (Fig. 8).


Fig. 1. Scaffold dieback	Fig. 2. Mycelial growth under bark	Fig. 3. Mycelial growth on roots	Fig. 4. A. tabescens mycelial mat

Fig. 5. A. mellea rhizomorphs	Fig. 6. Mushrooms on affected tree	Fig. 7. A. mellea (with annulus)	Fig. 8. A. tabescens (no annulus)

Disease Cycle

Armillaria spp. are widely distributed, affect numerous native forest trees, and persist in the soil in infected roots for many years. Replanting sites where infected trees have been removed is often not successful because of the difficulty in removing all old roots serving as an inoculum source for the newly planted trees. (Soil fumigation has generally not been successful in the Southeast.) Still, they do not cause significant tree loss in most orchards. They may become more important in high-density orchards because of the increased potential for spread from tree to tree through root contact.

Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by North Carolina Cooperative Extension nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension agent.

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Department of Plant Pathology
North Carolina State University
Raleigh, NC 27695 USA

Web Crafters: Anne S. Napier and Steve Schoof
Email: steve_schoof@ncsu.edu

Format updated March 29, 2011